The family of a retired US Federal Bureau of Investigation agent who went missing in Iran in 2007 has released a video appearing to show him being held hostage by unknown kidnappers.

Robert Levinson, a US citizen, appeared weary, thin but unharmed in the video, posted by his family on their website on Friday.

Levinson said he had been "treated well", but he "need[ed] the help of the United States government to answer the requests of the group that has held me for three and a half years".

The 54-second clip was accompanied by another video on the family's website, showing his wife and son pleading with his captors to let the 63-year-old father of seven go.

The video did not reveal where Levinson was being held or by whom. It was also not immediately clear when it was made. If Levinson's statement about it having been three and a half years since he was captured is accurate, the video would be approximately 15 months old.

Levinson appeared to be seated in front of a grey concrete wall in the video.

"I am not in very good health. I am running very quickly out of diabetes medicine," he said. "Please help me get home. Thirty-three years of service to the United States deserves something. Please help me."

A former US state department official who is familiar with the case said that the video was accompanied by a demand from Levinson's captors for the release of several prisoners.

Mysterious disappearance

Levinson's disappearance is shrouded in mystery. He went missing on Iran's Gulf island of Kish, which US citizens require no visa to visit, in March 2007.

Christine Levinson, the former US agent's wife, says her husband was in Kish as a private investigator, looking into cigarette counterfeiting in the region. She says he was last heard from on March 8, 2007.

US officials have been poring over the video for clues, and say that Levinson's remark that he was being held by a "group" suggested that he was not in the custody of a government.

Iran has denied detaining Levinson, and Hillary Clinton, the US secretary of state, had earlier this year said that the US had received information that he was being held in southwest Asia. She called on Iran to help locate him.

"If it has gotten to a point where the US government is really running out of options, maybe [the family] has to do things that haven't yet been done, such as going public and increasing the pressure on Iran," Reza Marashi, research director for the Washington-based National Iranian American Council, told the AFP news agency.

The Associated Press reported that the video had been sent to the family last year, but that it had not been released because the US government had advised them that this would complicate diplomatic efforts to return Levinson home.

It would appear that those efforts have stalled, and with worsening ties between Washington and Tehran, the family has sought to bypass diplomatic channels.

Not long after Clinton's remarks, the Levinson family received a series of photos of Levinson dressed in an orange prison jumpsuit like the ones worn by detainees at the US military prison at Guantanamo Bay, Cuba.